Re: [PATCH v2] sched/fair: Disable DL server on rcu_torture_disable_rt_throttle()

[Joel Fernandes <joelagnelf@nvidia.com>]

On Thu, Mar 06, 2025 at 09:35:27AM +0000, Juri Lelli wrote:
> Hi Joel,
> 
> On 05/03/25 20:10, Joel Fernandes wrote:
> > Currently, RCU boost testing in rcutorture is broken because it relies on
> > having RT throttling disabled. This means the test will always pass (or
> > rarely fail). This occurs because recently, RT throttling was replaced
> > by DL server which boosts CFS tasks even when rcutorture tried to
> > disable throttling (see rcu_torture_disable_rt_throttle()). However, the
> > systctl_sched_rt_runtime variable is not considered thus still allowing
> > RT tasks to be preempted by CFS tasks.
> > 
> > Therefore this patch prevents DL server from starting when RCU torture
> > sets the sysctl_sched_rt_runtime to -1.
> > 
> > With this patch, boosting in TREE09 fails reliably if RCU_BOOST=n.
> > 
> > Steven also mentioned that this could fix RT usecases where users do not
> > want DL server to be interfering.
> > 
> > Cc: stable@vger.kernel.org
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Fixes: cea5a3472ac4 ("sched/fair: Cleanup fair_server")
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> > v1->v2:
> > 	Updated Fixes tag (Steven)
> > 	Moved the stoppage of DL server to fair (Juri)
> 
> I think what I suggested/wondered (sorry if I wasn't clear) is that we
> might need a link between sched_rt_runtime and the fair_server per-cpu
> runtime under sched/debug (i.e., sched_fair_write(), etc), otherwise one
> can end up with DL server disabled and still non zero runtime on the
> debug interface. This is only if we want to make that link, though;
> which I am not entirely sure it is something we want to do, as we will
> be stuck with an old/legacy interface if we do. Peter?


While we are discussing, would it be acceptable to provide a sysctl that
disabled DL server? We could set this via boot parameters in rcutorture. That
would unblock the testing. If feel that is OK to do considering another
sysctl systctl_sched_rt_runtime is already there to disable RT throttling,
so we are just adding a similar one for DL server. What do you think?

thanks,

 - Joel